Method of growing crystalline bodies from the melt

ABSTRACT

The invention is an improved method for producing monocrystalline bodies of alumina (or other materials) that are characterized by varying cross-sections, for example, a sapphire tube having an internal flange. The bodies are grown from a thin film of melt, with the cross-section of the growing body being variable by varying the configuration of the film.

United States Patent Mlavsky et al.

[451- Feb. 25, 1975 METHOD OF GROWING CRYSTALLINE BODIES FROM THE MELTInventors: Abraham 1. Mlavsky, Lincoln;

Nicholas A. Pandiscio, Wayland,

both of Mass.

Assignee: Tyco Laboratories, Inc., Waltham,

Mass.

Filed: Aug. 6, 1973 Appl. No.: 386,175

Related US. Application Data Division of Ser. No. 148,589, June 1, 1971,abandoned.

US. Cl 23/301 SP, 23/305 Int. Cl B0lj 17/18 Field of Search 23/301 SP,273 SP, 305

[56] References Cited UNITED STATES PATENTS 3,801,309 4/1974 Mlavsky23/301 SP 3,826,625 7/1974 Bailey 73/301 SP Primary ExaminerJack SoferAssistant Examiner-D. Sanders Attorney, Agent, or FirmSchiller &Pandiscio [57] ABSTRACT The invention is an improved method forproducing mono-crystalline bodies of alumina (or other materials) thatare characterized by varying cross-sections, for example, a sapphiretube having an internal flange, The bodies are grown from a thin film ofmelt, with the cross-section of the growing body being variable byvarying the configuration of the film.

12 Claims, 8 Drawing Figures METHOD OF GROWING CRYSTALLINE BODIES FROMTHE MELT This application is a division of our copending application,Ser. No. 148,589 filed June 1, 1971 entitled Method of GrowingCrystalline Bodies from the Melt, now abandoned.

This invention relates to monocrystalline tubular bodies and moreparticularly to production of monocrystalline end walls or flanges onmonocrystalline tubes.

Various methods have been developed for growing monocrystalline bodiesfrom a melt. The present invention pertains to an improvement in growingcrystalline bodies from a melt according to what is called theedgedefined, film-fed, growth technique (also known as the EFG process).Details of this process are described in the copending US. Pat.Application of Harold E. La- Belle, .lr., Scr. No. 700126, filed Jan.24, 1968, now US Pat. No. 3,591,348 for Method of Growing CrystallineMaterials.

in the EFG process the shape of the crystalline body is determined bythe external or edge configuration of the end surface of a formingmember which for want of a better name is called a die. An advantage ofthe process is that bodies of selected shapes such as round tubes orflat ribbons can be produced commencing with the simplest of seedcrystal geometries, namely, a round small diameter seed crystal. Theprocess involves growth on a seed from a liquid film of feed materialsandwiched between the growing body and the end surface of the die, withthe liquid in the film being continuously replenished from a suitablemelt reservoir via one or more capillaries in the die member. Byappropriately controlling the pulling speed of the growing body and thetemperature of the liquid film, the film can be made to spread (underthe influence of the surface tension at its periphery) across the fullexpanse of the end surface of the die until it reaches the perimeter ofperimeters thereof formed by intersection of that surface with the sidesurface or surfaces of the die. The angle of intersection of theaforesaid surfaces of the die is such relative to the contact angle ofthe liquid film that the liquid's surface tension will prevent it fromoverrunning the edge or edges of the dies end surface. Preferably theangle of intersection is a right angle which is simplest to achieve andthus most practical to have. The growing body grows to the shape of thefilm which conforms to the edge configuration of the dies end surface.Since the liquid film has no way of discriminating between an outsideedge and an inside edge of the dies end surface, a continuous hole maybe grown in the crystalline body by providing in that surface a blindhole of the same shape as the hole desired in the growing body,provided, however, that any such hole in the dies end surface is madelarge enough so that surface tension will not cause the film around thehole to fill in over the hole. From the foregoing brief description itis believed clear that the. term edge-defined, film-fed growth denotesthe essential feature of the EFG process-the shape of the growingcrystalline body is defined by the edge configuration of the die andgrowth takes place from a film of liquid which is constantlyreplenished.

The primary object of the present invention is to provide an improvedmethod and apparatus for growing monocrystalline tubes using the EFGtechnique. An-

other object is to provide new monocrystalline tubular products.

In this connection it is known that the EFG process may be used to growmonocrystalline tubes of selected ceramic materials such as alumina andthat tubes made of monocrystalline or polycrystalline alumina haveutility as envelopes for high intensity vapor lamps. In the manufactureof such lamps the practice is to mount the electrodes in metal end capsthat are attached to the ends of the envelopes by brazing or othersuitable technique. It is recognized that mounting of the electrodes maybe facilitated by forming the tubes with end walls each having anopening for direct mounting of an electrode without need for an end cap.It is also desirable for other applications to form ceramic tubes eachhaving an imperforate end wall at one end or tubes that are integralextensions of solid rods of the same material or tubes having sectionsof different internal or external diameters. It also is desirable toform monocrystalline tubes on a continuous basis. Accordingly, a morespecific objectof this invention is to provide an improved method andapparatus for producing monocrystalline tubes of ceramic materials suchas alpha-alumina that terminate in integral end walls or flanges.

Another object is to provide a new and improved method of producingtubes of varying cross-sections.

Still another object is to provide a method of growing a monocrystallinebody comprising a series of connected tubular sections of likecross-section connected by transition sections whereby the body may besevered at said transition sections to form a plurality of short tubes.

Described briefly, the invention consists of providing a die or formingmember comprising a first member having a substantially horizontal endsurface adapted to be wet and used to support a film of melt from whicha crystal is to be pulled and one or more vertically extendingcapillaries that extend down from said end surface, a second membercoaxially disposed with respect to said first member and having acorrespondingly disposed end surface adapted to be wet by a film of thesame melt, and means providing relative movement of said first andsecond members in an axial direction so as to move said end surfacesinto and out of horizontal alignment with each other. A melt of thematerial to be crystallized is supplied to the lower end of thecapillaries and rises to the top ends by capillary action. Then a moltenfilm of melt is formed on the end surface of the first member so as toconnect with the melt in the capillaries and a crystal is grown from thefilm of melt.

I The film is caused to spread over the entire end surface of the firstmember and the pulling speed of the crystal and the temperature of thefilm are controlled so that the crystal grows from the film at allpoints along its entire horizontal expanse. After the crystal has grownto the desired length, the end surface of the second member is movedinto alignment with the end surface of the first member and the film iscaused to expand laterally over the entire end surface of the secondmember. The pulling speed of the crystal and the temperature of the filmare controlled so that the crystal now grows from the film overlyingboth end surfaces. Subsequently the end surface of the second member ismoved out of alignment with the end surface of the first member so thatcrystal growth can occur only from the film overlying the end surface ofthe first member. Additional melt is continuously supplied by thecapillaries to the film on the first end surface to replace the meltconsumed by crystal growth.

Other features and the advantages of the invention are described orrendered obvious by the following detailed description which is to beconsidered together with the accompanying drawings wherein:

FIG. 1 is an elevational view, partly in section, of apparatus forpracticing the method of this invention;

FIG. 2 is an elevational sectional view on an enlarged scale of aportion of the apparatus of FIG. 1;

FIG. 3 is a plane view of a part of the apparatus of FIG. 2;

FIG. 4 is a view similar to FIG. 2 showing one phase of the growthprocess;

FIGS. 5 and 6 are views similar to FIG. 3 showing other phases of thegrowth process;

FIG. 7 illustrates one form of tube that can be produced according tothe invention; and

FIG. 8 shows apparatus for a modification of the invention.

The present invention may be used to produce monocrystalline tubes madeof any one of a variety of congruently melting materials that solidifyin identifiable crystal lattices. By way of example, the material may bealumina, barium titanate, lithium niobate and yttrium aluminum garnet.The invention is also applicable to other materials, notably materialsthat melt congrently (i.e., compounds that melt to a liquid of the samecomposition at an invariant temperature). The following detaileddescription of the invention is directed to growing tubes of sapphire,i.e., monocrystalline alphaalumina.

FIG. 1 shows a furnace embodying the invention. The furnace consists ofavertically moveable horizontal bed 2 which engages a stationary furnaceenclosure consisting of two concentricspaced quartz tubes 4 and 6 thatare supported at their opposite ends in two annular heads 8 and 10 thatseal off the space between the two tubes. The bottom end of the innertube 4 extends below bottom head 8 and is positioned in a gasket 12disposed in a cavity in the bed. The bottom head 8 is provided with aninlet port fitted with a pipe 14. The upper head 10 has an outlet portwith a pipe 16. Pipes 14 and 16 are connected to a pump (not shown) thatcontinuously circulates cooling water through the space between the twoquartz tubes. The circulating water not only keeps the inner quartz tubeat a safe temperature but also absorbs most of the infrared energy andthereby makes visual observation of crystal growth more comfortable tothe observer. The interior of the furnace enclosure is connected by apipe 18 mounted in the bed 2 to a vacuum pump or to a regulated source(not shown) of inert gas such as argon or helium. The furnace enclosurealso is surrounded by an RF heating coil 20 that is coupled to acontrollable 500kc. power supply (not shown) of conventionalconstruction. The heating coil may be moved up and down along the lengthof the furnance enclosure and means (not shown) are provided forsupporting the coil at a selected elevation.

The head 10 is attached and supported by a conventional crystal pullingmechanism represented schematically at 22. The crystal pulling mechanism22 has an elongate pulling rod 24 that extends through the head 10 andinto the furnace enclosure. It is to be noted thatv the type ofcrystal-pulling mechanism is not critical to the invention and that theconstruction thereof may be varied substantially. Preferably, however,we prefer to employ a crystal pulling mechanism that is hydraulicallycontrolled since it offers the advantage of being vibration-free andproviding a uniform pulling speed. Regardless of its exact constructionwhich is not required to be described in detail, it is to be understoodthat the pulling mechanism 22 is adapted to move pulling rod 24 axiallyat a controlled rate. Pulling rod 24 is disposed coaxially with thequartz tubes 4 and 6 and its lower end has an extension in the form ofmetal holder 26 that is adapted to releasably hold a seed on whichcrystal growth is made to-occur as hereafter described. By way ofexample, the seed may be a monocrystalline tube 28 grown previously bythe EFG technique.

Located within the furnace enclosure is a cylindrical heat susceptor 30made of carbon. The top end of susceptor 30 is open but its bottom endis closed off by an end wall. The susceptor is secured to and supportedby a plurality of tungsten rods 32 that are anchored in bed 2. Supportedwithin susceptor 30 on a plurality of short tungsten rods 34 is acrucible 36 adapted to contain a melt 38 of the material to be grown inaccordance with the invention. Rods 34 are secured to susceptor 30 andcrucible 36 so as to prevent movement of the crucible. The crucible ismade of a material that will withstand the operating temperatures andwill not react with or dissolve in the melt. With an alumina melt, thecrucible is made of molybdenum but it also may be made of tungsten,iridium or some other material with similar properties with respect tomolten alumina. Where a molybdenum crucible is used, it must be spacedfrom the susceptor since there is a eutectic reaction between carbon andmolybdenum at about 2,200C. The inside of the crucible is of suitablesize and shape, preferably with a constant diameter. To help obtain thehigh operating temperatures necessary for the process, a cylindricalradiation shield 40 made of carbon cloth may be wrapped around thecarbon susceptor. The carbon cloth greatly reduces the heat loss fromthe carbon susceptor.

Referring now to FIGS. l-3, mounted in crucible 36 is a die assemblyidentified generally by the numeral 44. The die assembly is made ofmolybdenum and comprises a cylindrical sleeve 46 that is affixed (e.g.by welding or press fit) to a supporting disc 48 that is locked to thecrucible. The bottom end of sleeve 46 is welded to the bottom wall ofthe crucible so as to prevent leakage of melt to the interior of thesleeve. Sleeve 46 has a plurality of axially extending,circumferentially spaced, circular bores 54 and radial openings 56 nearits bottom end to permit inflow of melt to the several bores from thecrucible. Bores 54 are sized to function as capillaries for moltenalumina. The upper end of sleeve 46 terminates in a flat horizontalsurface 58 which intersects the sleeves outer surface at a right angle.It is to be noted that sleeve 46 projects above disc 48 so as to bevisible to the operator. The length of the sleeve 46 and diameter of thecapillaries 54 are such that molten alumina can rise in and fully fillthe capillaries by action of capillary rise so long as the level of themelt in the crucible is high enough to flow into the openings 56. Theheight to which a column of melt can rise by capillary action in one ofthe capillaries 54 can be approximated by the equation h=2Tcost/drg,where h is the distance in cm. that the column will rise; T is thesurface tension of the melt in dynes/cm.; L is the contact angle of themelt; d is the density of the melt,

r is the internal radius of the capillary in cm.; and g is thegravitational constant in cm/sec By way of example in a capillary of0.75 mm. diameter in a molybdenum member, a column of molten alumina maybe expected to rise more than 1 1 cm. by capillary action.

Slidably disposed with sleeve 46 is a molybdenum rod 60 of circularcross-section. The upper end of rod 60 terminates in a flat horizontalsurface 62 having an axially-extending cavity 64. The latter has adiameter large enough so that surface tension will not cause a film ofmelt on surface 62 to fill in over it. In this connection it is to benoted that end surface 62 intersects the outer surface of rod 60 andalso the cylindrical surface forming the side wall of cavity 64 at aright angle. Rod 60 is sized so as to make a snug sliding fit withsleeve 46, particularly at the temperature (about 2,070C) at which thedie assembly is maintained during crystal growth. Rod 60 extends througha hole 68 in the bottom of the crucible and also through a hole 70 inthe bottom wall of susceptor 30. Hole 70 is oversized so as to preventreaction of the molybdenum rod and carbon susceptor.

Referring now to FIG. I, the bottom end of rod 60 is connected by acoupling 72 to a larger diameter rod 74 that is slidably mounted in asleeve bearing 76 that is secured in the bed 2. The lower end of rod 74is connected by a second coupling 78 to the piston rod 80 of a hydraulicactuator 82. The latter is of the doubleacting type, having two inletports 84 and 86 at opposite ends of its cylinder. Ports 84 and 86 areconnected by hose lines 88 to a suitable source of pressurized hydraulicfluid (not shown) via a suitable reversing valve shown schematically at90. Valve 90 is adapted to selectively apply fluid under pressure toeither of ports 84 and 86. When fluid pressure is applied via inlet port84, piston rod 80 retracts into the actuator cylinder. Application offluid pressure to inlet port 86 causes piston rod 80 to be extended.Actuator 82 is mounted on a supporting bracket 92 that is secured to thebed 2. The bed 2 is mounted on a pair of vertical slide rods 94 that areattached to a supporting framework (not shown) that also supports thepulling mechanism. Additionally the bed 2 is supported by a mechanism(not shown) that is adapted to lower and raise the bed and hold it at aselected height, Such bed raising and lowering mechanisms are well knownin the art of crystal growing furnaces and, therefore, need not be shownin detail. Preferably, however, the bed raising and lowering mechanismis hydraulically operated. The apparatus just described is designed topermit growth of tubular crystal bodies that are characterized by spacedinternal flanges, i.e., tubes that comprise successive tubular sectionsof constant inner and outer diameters connected by shorter tubularsections having the same outer but a different inner diameter. For wantof a better name, the latter sections may be termed transition sections.The same apparatus may be used to grow a tube of one wall thickness ontoa tube of a different wall thickness. Crystal growth may be initiatedusing a tubular or nontubular seed. Thus. it is possible to start withan alphaalumina seed in the form of a monocrystalline filament or ribbonand grow a tube onto the seed in accordance with the EFG techniquedescribed in copending application Ser. No. 700l26, now U.S. Pat. No.3.59l.348 of Harold E. LaBelle, .lr. Preferably, however, it ispreferred to use a monocrystalline tube previously grown diameter outersurface and a stepped inner surface may be grown according to theinvention using the apparatus of FIGS. 1 and 2. It is to be noted thatgrowth may be initiated with rod 60 disposed either in lowered position(FIG. 4) or raised position (FIG. 5). Assuming for purpose ofexplanation that initially rod 60 is in the lowered position shown inFIG. 4 and the crucible and capillaries are filled with an alumina melt,a previously grown sapphire tube 28 is mounted in holder 26 in axialalignment with the die assembly. Tube 28 has substantially the same 0.d.and i.d. as the die surface 58. Then with the power input to coiladjusted so that the upper end surface of 58 of sleeve 46 is aboutl0-40C higher than the melting point of the tube 28, the tube is loweredinto contact with the surface 58 and held there long enough for aportion of the end of the tube to melt and form a liquid film 96 thatconnects with the melt in one or more of the capillaries and preferablycovers all of the surface 58. It is to be noted that the capillaries arefilled with melt but are shown empty in FIGS. 4-6 in order to render thecapillaries more distinct to the reader. Further it is to be understoodwith reference to FIG. 4 that before the end of tube 28 is melted toform film 96, the melt in each capillary has a concave meniscus with theedge of the meniscus being substantially flush with surface 58. Thetemperature gradient along the length of the tube and the temperature ofsurface 58 are factors influencing how much of the tube melts and thethickness of the film 96. In this connection it is to be noted that thetube functions as a heat sink so that its temperature is lower atsuccessively higher points thereon. However, the thermal gradient alongtube 28 is affected by the height of coil 20 and susceptor 30 and alsothe power input to the coil. In practice these parameters are adjustedso that the initial film 96 has a thickness in the order of 0.1mm.

Once the film 96 has connected with melt in at least one of thecapillaries, the pulling mechanism 34 is actuated to pull tube 28upwardly away from surface 58. The pulling speed is set so that surfacetension will cause the film to adhere to the tube long enough forcrystallization to occur due to a drop in temperature at the solidtube-liquid film interface. The drop in temperature occurs because ofmovement of the tube away from the surface 58, i.e. because thesolid-liquid interface sees a lower temperature. If the initial filmdoes not fully cover surface 58, as tube 28 is pulled surface tensionwill cause the film to spread fully over surface 58 (see FIG. 3). Thus,as tube 28 is pulled, crystal growth will occur at all points along thehorizontal expanse of the film with the result that a tubularmonocrystalline extension is formed on the tube which has substantiallythe same cross-sectional shape and size as the tube. The film consumedby the crystal growth is replaced by additional melt which is suppliedby the capillaries 54. The process is continued until the tubularmonocrystalline extension has growm to a desired length. Then actuator82 is operated to raise rod just enough to place its top and surface 62flush with end surface 58. Once this has occurred, surface tension willcause film 96 to spread radially inward over end sur face 62. Adjustmentof the pulling speed and/0r operating temperature may be required inorder to cause the result that the newly grown crystal will havesubstantially the same outer diameter as surface 58 and an innerdiameter 98 approximately the same as that of cavity 64 (see FIG. Thechange in inside diameter of the tube is not sharp but tapered as shownat 99 in FIG. 5. Thereafter the growth may be continued without furtherchange in position of rod 60, in which case the product will comprise atube having a first section with a relatively large i.d. and a secondsection with a relatively small i.d. while its 0.:1. will besubstantially constant.

It is also possible to start the growth process with tube 60 retractedbut using as a seed a previously grown tube 28A having the same 0.d. assleeve 58 and an i.d. equal to the diameter of cavity 64. In this casethe film formed by melting the seed tube will cover only the uppersurface 58 of sleeve 46. Accordingly the crystal growth produced on theseed tube as it is being pulled as above described forms a tubularextension having the same o.d. as the original tube, but an i.d. asshown at 97 that is about the same as the i.d. of sleeve 46.

It is contemplated also that rod 60 may be repetitively raised andlowered at selected intervals during the growth process, in which'casethe product will have alternately occurring sections 100 and 102 ofrelatively large and relatively small internal diameter (FIG. 7). Thisproduct may be cut into shorter lengths at convenicnt points alongeither the sections 100 or the sections 102.

In the case of providing envelopes for lamps, it is preferable to cutthe product at the transition sections 102. Accordingly the sections 102are made long enough so that when severed, e.g. along line 104, to forma plurality of discrete tubes, each tube will have an internal flange(which may also be considered as end wall with a center hole) that isthick enough to provide the rigidity required for it to function as asupport for a lamp electrode.

It is to be noted that the pulling speed and the temperature of the filmmay be varied during the crystal growth. However, the pulling speedshould not be so great and the film temperature so high as to cause thetube to pull free of the melt film. In growing alphaalumina, it ispreferred to have an initial pulling speed of about 0.1 in/min until itis determined that the film fully covers the supporting end surface andto thereafter increase the speed to about 0.2 in/min. The pulling speedand the film temperature control the thickness of the film which alsocontrols the rate at which the film will spread. Within limits, the filmthickness can be increased by increasing the film temperature and thepulling speed.

The following example illustrates a preferred mode of practicing theinvention.

EXAMPLE I A molybdenum crucible having an internal diameter of about1.50 inch, a wall thickness of about 0.20 inch, and an internal depth ofabout 0.60 in. is positioned in the furnace in the manner shown inFIG. 1. Disposed in the crucible is a die assembly constructed generallyas shown in FIG. 2. The sleeve 46 has four capillaries 54 spaceduniformly about its axis. The upper end surface 58 of sleeve 46 has anoutside diameter of about 0.500 inch and an inside diameter of about0.450 inch. The length of sleeve 46 is such that its upper end projectsabout one-sixteenth inch above the crucible. The rod 60 has an outsidediameter of about 0.445 inch and its cavity 64 has a diameter of about0.31 inch. The actuator is adapted to move rod 60 through a stroke ofabout 0.30 inch between upper and lower limit positions. In the upperlimit position, its upper end surface 62 is flush with end surface 58.The four capillaries each have a diameter of about 0.03 inch. Thecrucible is filled with substantially pure polycrystalline alphaaluminaand a monocrystalline alpha-alumina tube 28 grown previously by the EFGtechnique is mounted in holder 26. Tube 28 is cylindrical and was grownso that the c-axis of its crystal lattice extends parallel to itsgeometric axis. Additionally, tube 28 has substantially .the same insideand outside diameter as surface 58 of sleeve 46. Tube 28 is mounted inholder 26 so that it is aligned with surface 58. Access to seed holder26 and the crucible 36 is achieved by lowering bed 2 away from thefurnace enclosure and lowering the seed holder below the bottom end offurnace tube 4. With the bed restored to the position of FIG. 1, rod 60is lowered to its lower limit position (FIG. 4). Cooling water isintroduced between quartz tubes 4 and 6 and the furnace enclosure isevacuated and filled with argon to a pressure of about one atmospherewhich is maintained during the growth period. Then the R.F. coil 20 isenergized and operated so that alumina in the crucible is brought to amolten condition (alumina has a melting point in the vicinity of 2,050C)and the surface 68 reaches a temperature of about 2,070C. As the solidalumina is converted to the melt 38, columns of the melt will rise inand fill capillaries 54. Each column of melt will rise until itsmeniscus is substantially flush with the top of the rod. After affordingtime for temperature equilibrium to be established, the pullingmechanism is actuated and operated so that the tube 28 is moved intocontact with the upper surface 58 of the die assembly and allowed torest in that position long enough to allow the bottom end of the tube tomelt and form film 96, After about 60 seconds, the tube is with drawnvertically at the rate of about 0.1 inch per minute. As the tube iswithdrawn, crystal growth will occur on the seed and at the same time,if it does not already fully coversurface 58, the film 96 will spreadfully over the surface 68 due to its affinity with the newly grownmaterial on the tube and the films surface tension. The latter forcealso causes additional melt to flow out of the capillaries and add tothe total volume of film.

As the tube 28 is pulled, the crystal growth will propagate verticallythroughout the entire horizontal expanse ofthe film 96, with the resultthat growing crystal will conform in cross-sectional shape to thesurface 58. At this point the pulling speed is increased to about 0.2inch/min. and the temperature of the surface 58 held constant at about2,070C. Growth is continued until a monocrystalline tubular extension ofabout 4 inches has been produced on the seed tube. Then, as pullingcontinues, actuator 82 is operated to raise rod 60 to its upper limitposition (FIG. 5) so as to place its surface 62 even with surface 58.Once this has been done, the film 96 will begin to spread onto surface62. Spreading of the film is helped by raising the temperature ofsurface 58 to about 2,080C. As the film begins to spread radially inwardover surface 62, the crystal growth will also expand horizontally. Thefilm stops spreading when it reaches the edge of cavity 64, and aspulling continues the crystal growth will propagate verticallythroughout the entire horizontal expanse of the expanded film, with theresult that the growing crystal will now have the same 0.d. as sleeve46, but an i.d. as shown at 98 (FIG. Crystal growth is continued untilthat portion of the tubular extension having the reduced diameter 98reaches a length of one-half inch, whereupon the pulling speed isincreased to about 1.0 inch per minute. At this higher pulling speed,the crystal body pulls free of the melt film. Thereafter, the pullingmechanism is stopped and the furnace cooled. The tube 28 is retrievedfrom holder 26. The grown body is found to be substantiallymonocrystalline and a crystallographic extension of the crystal latticeof the seed tube 28. Its outside diameter is substantially constant andapproximately the same as the 0.d. of sleeve 46. Its inner surfacecomprises a long section (about 4 inches) with a diameter about the sameas the i.d. of sleeve 46, and two shorter sections, one having adiameter of about the same as cavity 64 and the other being tapered asshown at 99. The inner and outer surfaces are both smooth.

EXAMPLE ll In this example, the seed tube and the procedure are the sameas in Example I, except that growth is not terminated after that portionof the tubular extension having the reduced diameter 98 has reached alength of about one-half inch. Instead that portion is allowed to growto a length of about three-fourths inch, whereupon rod 60 is lowered toits original position (FIG. 4). Rod 60 is lowered at a rate such that asit drops the surface tension will cause film 96 to recede radially awayfrom cavity 64 until surface 62 is substantially completely free of meltfilm. Rod 60 preferably is lowered at a rate in the order of the pullingspeed of tube 28. As the film recedes from surface 62, the crystalgrowth also decreases horizontally while continuing to propagatevertically from the film, with the result that the growing body has agradually expanding internal diameter. Once the film has returned to theinternal edge of surface 58, it stops shrinking and now the crystal bodycontinues to grow vertically with substantially the same 0.d. and i.d.as sleeve 46. The crystal is allowed to grow an additional 4 inches andthen rod 60 is again elevated to the position shown in FIG. 5, whereuponthe film again expands over the surface 62 and the crystal again growsto the diameter of cavity 64. Thereafter rod 60 is repeatedly loweredand raised to repeatedly vary the internal diameter of the product asabove described. The result is a product having a stepped internal wallas shown in FIG. 7.

Although a monocrystalline tube has been used as the seed to initiatecrystal growth, it also is possible to start with a seed of some othershape, e.g. a monocrystalline ribbon or filament, and grow a tubetherefrom as described in the aforesaid copending application of HaroldE. LaBelle, Jr. Once the body has reached a tubular shape, it may begrown so as to have a stepped internal surface as herein described.

It is to be noted that the invention may be used in growing tubular orrod extensions (or flanges or end walls) of other cross-sectionalshapes, e.g. rectangular. square, triangular. etc. on tubes or rods ofthe same or different cross-sections. Thus, for example, by making thecross-section of rod 60 and cavity 64 and the inner edge configurationof sleeve 46 square, it is possible to grow a tubular extension ortermination of square interior shape and round exterior shape onto atube of round or square cross-section. By eliminating cavity 64 andpositioning rod 60 as in FIG. 5, it is possible to grow a solid rod ontoa tube. Also, by way of example, by making cavity 64 triangular orhexagonal and the outer edge configuration of sleeve 46 square, it ispossible to grow a tubular extension with a cross-sectionalconfiguration that is triangular or hexagonal on the interior and squareon the exterior.

An important advantage of the invention is that it is applicable tocrystalline materials other than alumina. It is notlimited tocongruently melting materials and encompasses growth of materials thatsolidify in cubis, rhombohedral, hexagonal and tetragonal crystalstructures, including barium titanate, yttrium aluminum garnet, andlithium niobate mentioned above. With respect to such other materials,the process is essentially the same as that described above foralpha-alumina, except that it requires different operating temperaturesbecause of different melting points. Additionally, certain minor changesmay be required in the apparatus, e.g., different crucible materials inorder to avoid reaction between the melt and the crucible.

Laue X-ray back reflection photographs of alphaalumina crystal growthproduced according to the foregoing invention reveals that the crystalgrowth usually comprises one or two, and in some cases three or four,crystals growing together longitudinally separated by a low angle(usually within 4 of the c-direction) grain boundary. Therefore, forconvenience and in the interest of avoiding any suggestion that thecrystal growth is polycrystalline in character, we prefer to describe itas substantially monocrystalline, it being understood this term isintended to embrace a crystalline body that is comprised of a singlecrystal or two or more crystals, e.g., a bicrystal or tricrystal,growing together longitudinally but separated by a relatively smallangle (i.e. less than about 4 grain boundary. The same term is used todenote the crystallographic nature of the seed tube.

It also has been found that best results are achieved if the c-axis ofthe crystal lattice of the seed tube extends parallel to the tubeslongitudinal axis, so that the extension forming a flange or end wallalso grows vertically along the c-axis. Growth in the c-direction ischaracterized by smooth surfaces and superior strength.

Obviously the invention is susceptible of modification and may bepracticed otherwise than as specifically described above. For example,it is possible to grow tubular rods having a varying externalcrosssectional configuration, e.g., a tube having spaced externalflanges rather than internal flanges. This may be accomplished by usinga die assembly having a close fitting sleeve slidably disposed aroundthe outside of a stationary die member having one or more capillaries,and means for raising and lowering the sleeve in the same manner as rod60. FlG. 8 illustrates one possible die arrangement for growing tubularbodies with varying external cross-sectional configurations. In thiscase the die assembly consists of a cylindrical rod 108 having a flattop end surface 110 with a cavity 112 (which serves the same purpose ascavity 64) and a plurality of capillaries in the form of through bores114. The bottom end of rod '108 has side openings 116 to admit melt tothe capillaries. Rod 108 also has a large center bore 118 in which isslidably disposed a slide rod 120 that extends through a hole in thebottom wall 122 of crucible 36. Although not shown, it is to beunderstood that the bottom end of slide rod 120 is connected tooperating rod 72 so that it may be raised or lowered by actuator 82. Rod108 also has two diametrically opposed slots 124 in that portion thereofthat defines bore 118. Slots 124 are wide enough to slidably accommodateportions of a pin 126 that is carried by slide rod 120. The ends of pin126 are anchored in a cylindrical sleeve 128 that surrounds and makes aclose sliding fit with the rod 108. The upper end of sleeve 128 has aflat annular surface 130. The upper end of the crucible is covered by aremoveable disc 132 that functions as a radiation shield for the meltand has a center hole to slidably accommodate sleeve 128. It is to beunderstood that actuator 82 can move sleeve 128 from a suitably lowerlimit position such as shown in FIG. 8 to an upper limit position inwhich its annular end surface 130 is flush with the upper annular endsurface 110. When sleeve 128 is in its lower limit position, crystalgrowth occurs from a film of melt supported by the end surface 110. Whensleeve 128 is in its upper limit position, the film of melt can be madeto cover its end surface 130 as well as surface 110 and crystal growthoccurs from this larger film of melt. In other words, when sleeve 128 isdown, the crystal growth will conform in crosssectional configuration tothe annular shape of surface 110, whereas when sleeve 128 is raised, thecrystal growth will have an exterior diameter close to the diameter ofcavity 112. Leakage of melt via the hole pro vided in the bottom of thecrucible for rod 120 is avoided by welding the bottom end of rod 108 tothe crucibles bottom wall. Initial formation of a film of melt on theupper end surface III) of the capillary assembly may be achieved in amanner similar to that described above in connection with Example I. butpreferably using as a seed a previously grown monocrystalline tubehaving an 0.d. no greater than the 0.d. of end surface 110. If the meltand seed tube are alumina, the operating temperature and pulling speedare the same as in Example I and outward expansion of the melt film ontosurface 130 is conducted by controlling the operating temperature andpulling speed in the same manner as employed in Example I to achieveinward expansion from surface 58 onto surface 62. The apparatus of FIG.8 may be used to grow a tubular or solid rod extension (or an exteriorflange) of a relatively large outside diameter on a tube or rod ofrelatively small outside diameter and as with the die assembly of FIG.2, the die assembly of FIG. 8 may be modified so as ,to grow bodies ofround, square, rectangular or other like cross-sectional configurations.In this connection it is to be noted that a solid rod may be grown byomitting cavity 112.

With respect to the die assembly, it is to be understood that in thefollowing claims the term surface" as it pertains to a die member isintended to cover the effective film-supporting surface of that diemember, whether the member is made ofa single piece or as two or morepieces, and the term "capillary" is intended to denote a passageway thatcan take a variety of forms. In this connection it is to be noted thatthe sleeve 46 of FIG. 2 may actually consist of two concentric spacedsleeves locked against relative movement and spaced uniformly so as toprovide a continuous annular space therebetween that is adapted tofunction as one large capillary. Thus, in the die assembly of FIG. 2,sleeve 46 may be replaced by two round sleeves locked to each other inconcentric spaced relation, with the annular space therebetweenmeasuring about 0.03 inch in a radial direction. Of course, since theannular space functions as a capillary, the two sleeves need not havebores like those shown at 54, but the outer sleeve must have openings atits bottom end (corresponding to openings 54) to permit inflow of meltto the annular capillary. The rod would be disposed within the inner oneof the concentric sleeves. The effective film supporting surface of adie member having one or more capillaries is understood to be its entireend surface considered as if the capillary orifices were not presentsince when a film of melt fully overlies the end surface it covers overthe capillary orifices.

What is claimed is:

1. Method of growing a crystalline body of a selected material from amelt thereof comprising establishing a molten film of said materialoverlying and covering a first substantially horizontal surface ofpredetermined configuration, growing and pulling a crystalline body fromsaid film, moving a second substantially horizontal surface intocontiguous and edge-aligned relation with said first surface after saidbody has grown to a selected length, spreading said film so that itoverlies both said surfaces, growing and pulling said body from the filmoverlying both said surfaces, and continuously replenishing said film assaid body is being grown and pulled.

2. Method of claim 1 wherein said film is replenished by feeding a meltof said material to an opening in said first surface.

3. Method ofclaim 2 wherein said film is replenished by feeding melt tosaid opening from a crucible.

4. Method of claim I wherein said body is grown with a cross-sectioncorresponding to the full-horizontal expanse of said film.

5. Method of claim 1 further including the step of withdrawing saidsecond surface out of contiguous and edge-aligned relation with saidfirst surface, and continuing to grow and pull said body only from thefilm overlying said first surface.

6. Method of claim 1 wherein said second surface has an outside diameterthat is smaller than the inside diameter of said first surface.

7. Method of producing a monocrystalline tube of a selected materialfrom a melt of said material comprising disposing in a crucible a memberhaving a first annular end surface and at least one capillary leadingdown from said annular surface into said crucible, and a second memberdisposed within said first member havibng a second annular end surface,providing in said crucible a melt of said material in a quantitysufficient to flow up said at least one capillary by action of capillaryrise, establishing on said first annular end surface a molten film ofsaid material that extends to and is continuous with the melt in saidcapillary, growing and pulling from said film a substantiallymono-crystalline tube of said material having a cross sectioncorresponding substantially to the configuration of said first endsurface and simultaneously replenishing said film with melt in saidcapillary, moving said second member to place said second end surface inclose-fitting and flush relation with said first end surface, spreadingsaid film so that it overlies both said end surfaces and growing saidtube so that it has an outer edge configuration like that of said secondend surface.

8. Method of claim 7 further including moving said second member toplace said second end surface below the level of said first end surfaceand thereafter growing said tube so that it has a cross sectioncorresponding substantially to said first end surface.

9. Method of producing a substantially monocrystalline tube of aselected material so that said tube includes spaced flanges, comprisingestablishing a molten film of said material on an annular end surfaceofa first fixed member so that said film substantially fully covers saidend surface, growing and pulling from said film a monocrystalline bodyof said material having a configuration in cross section correspondingsubstantially to the edge configuration of said end surface,continuously replenishing said film by feeding a melt of said materialto said end surface from a crucible via an orifice in said end surface,placing another annular end surface of a second moveable member inclose-fitting and flush relation with the annular end surface of saidfirst member, expanding said film so that it substantially fully coversboth said end surfaces, and continuing to grow and pull said body sothat at its interface with said expanded film it grows to a crosssection having an outer diameter substantially equal to the outerdiameter of one end surface and an inner diameter substantially equal tothe inner diameter of the other end surface, thereafter moving the endsurface of said second member out of flush relation with the end surfaceof said fixed member so that said growing body has an interface onlywith the film on the end surface of said first member, continuing togrow and pull said body so that at its interface with the film it has across sectional configuration corresponding generally to the grossconfiguration of the end surface of said first member, and repeating theforegoing steps commencing with the step of placing the end surface ofsaid second member in flush relation with the end surface of said firstmember.

10. Method of claim 9 wherein the end surface of said second member ismoved into and out of flush relation with the end surface of said firstmember by reciprocating said second member vertically relative to saidfirst member.

11. Method of claim 1 wherein said second surface is concentric and hasa larger diameter than said first mina.

1. METHOD OF GROWING A CRYSTALLINE BODY OF A SELECTED MATERIAL FROM AMELT THEREOF COMPRISING ESTABLISHING A MOLTEN FILM OF SAID MATERIALOVERLYING AND VOVERING A FIRST SUBSTANTIALLY HORIZONTAL SURFACE OFPREDETERMINED CONFIGURATION, GROWNING AND PULLING A CRYSTALLINE BODYFORM SAID FILM, MOVING A SECOND SUBSTANTIALLY HORIZONTAL SURFACE INTOCONTIGUOUS AND EDGEALIGNED RELATION WITH SAID FIRST SURFACE AFTER SAIDBODY HAS GROWN TO A SELECTED LENGTH, SPREADING SAID FILM SO THAT ITOVERLIES BOTH SAID SURFACES, GROWING AND PULLING SAID BODY FORM THE FILMOVERLYING BOTH SAID SURFACES, AND CONTINOUSLY REPLENISHING SAID FILM ASSAID BODY IS BEING GROWN AND PULLED.
 2. Method of claim 1 wherein saidfilm is replenished by feeding a melt of said material to an opening insaid first surface.
 3. Method of claim 2 wherein said film isreplenished by feeding melt to said opening from a crucible.
 4. Methodof claim 1 wherein said body is grown with a cross-section correspondingto the full-horizontal expanse of said film.
 5. Method of claim 1further including the step of withdrawing said second surface out ofcontiguous and edge-aligned relation with said first surface, andcontinuing to grow and pull said body only from the film overlying saidfirst surface.
 6. Method of claim 1 wherein said second surface has anoutside diameter that is smaller than the inside diameter of said firstsurface.
 7. Method of producing a monocrystalline tube of a selectedmaterial from a melt of said material comprising disposing in a cruciblea member having a first annular end surface and at least one capillaryleading down from said annular surface into said crucible, and a secondmember disposed within said first member havibng a second annular endsurface, providing in said crucible a melt of said material in aquantity sufficient to flow up said at least one capillary by action ofcapillary rise, establishing on said first annular end surface a moltenfilm of said material that extends to and is continuous with the melt insaid capillary, growing and pulling from said film a substantiallymono-crystalline tube of said material having a cross sectioncorresponding substantially to the configuration of said first endsurface and simultaneously replenishing said film with melt in saidcapillary, moving said second member to place said second end surface inclose-fitting and flush relation with said first end surface, spreadingsaid film so that it overlies both said end surfaces and growing saidtube so that it has an outer edge configuration like that of said secondend surface.
 8. Method of claim 7 further including moving said secondmember to place said second end surface below the level of said firstend surface and thereafter growing said tube so that it has a crosssection corresponding substantially to said first end surface.
 9. Methodof producing a subStantially monocrystalline tube of a selected materialso that said tube includes spaced flanges, comprising establishing amolten film of said material on an annular end surface of a first fixedmember so that said film substantially fully covers said end surface,growing and pulling from said film a monocrystalline body of saidmaterial having a configuration in cross section correspondingsubstantially to the edge configuration of said end surface,continuously replenishing said film by feeding a melt of said materialto said end surface from a crucible via an orifice in said end surface,placing another annular end surface of a second moveable member inclose-fitting and flush relation with the annular end surface of saidfirst member, expanding said film so that it substantially fully coversboth said end surfaces, and continuing to grow and pull said body sothat at its interface with said expanded film it grows to a crosssection having an outer diameter substantially equal to the outerdiameter of one end surface and an inner diameter substantially equal tothe inner diameter of the other end surface, thereafter moving the endsurface of said second member out of flush relation with the end surfaceof said fixed member so that said growing body has an interface onlywith the film on the end surface of said first member, continuing togrow and pull said body so that at its interface with the film it has across sectional configuration corresponding generally to the grossconfiguration of the end surface of said first member, and repeating theforegoing steps commencing with the step of placing the end surface ofsaid second member in flush relation with the end surface of said firstmember.
 10. Method of claim 9 wherein the end surface of said secondmember is moved into and out of flush relation with the end surface ofsaid first member by reciprocating said second member verticallyrelative to said first member.
 11. Method of claim 1 wherein said secondsurface is concentric and has a larger diameter than said first surface.12. Method of claim 1 wherein said material is alumina.